Fiber optic networks are becoming increasingly popular for data transmission due to their high speed, high capacity capabilities. A common and well known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect where the channels within a signal travel through an optic fiber at different speeds, i.e., longer wavelengths travel faster than shorter wavelengths. This problem becomes more acute for data transmission speeds higher than 2.5 gigabytes per second. The resulting pulses of the signal will be stretched, possibly overlap, making it more difficult for a receiver to distinguish where one pulse begins and another ends. This seriously compromises the integrity of the signal. Therefore, for a fiber optic communications system to provide a high transmission capacity, the fiber optic communications system must compensate for chromatic dispersion.
The exact value of the chromatic dispersion produced in a channel of a wavelength-division multiplexed fiber optic communications system depends upon several factors, including the type of fiber and the wavelength of the channel. FIG. 1 illustrates the graphs of Group Velocity Dispersion, D, against wavelength, of three conventional fiber transmission bands, or transmission windows, and conventional fiber types which operate in these bands. For example, single-mode or non-dispersion shifted fibers (NDSF) operate in the 1.3 .mu.m region, and dispersion shifted fibers (DSF) are optimized for single channel signal transmission in the C-Band. With the development of wavelength-division multiplexed (WDM) fiber optic communications systems, where several signal channels, each channel comprising a different wavelength band within the C-Band, are simultaneously propagated along a span of an individual fiber, "non-zero dispersion shifted fibers" (NZ-DSF) were developed. The NZ-DSF has zero-dispersion at the edge of or outside of the C-Band, and moderately low non-zero dispersion in the region of the C-Band.
Because all three fiber types are deployed in telecommunications systems, the requirements for dispersion compensators vary widely. A constant level of dispersion compensation does not accurately negate the dispersion of all channels. This inaccuracy can become a significant problem for high-speed data propagation, long span distances, and/or wide distances between the shortest and longest wavelength channels.
Some conventional dispersion compensators attempt to solve this problem, including dispersion compensation fibers, chirped fiber Bragg gratings coupled to optical circulators, and conventional diffraction gratings disposed as sequential pairs.
A dispersion compensation fiber, which is used in-line within a fiber communications system, has a special cross-section index profile so as to provide chromatic dispersion that is opposite to that of ordinary fiber within the system. The summation of the two opposite types of dispersion negates the chromatic dispersion of the system. However, dispersion compensation fiber is expensive to manufacture, has a relatively large optical attenuation, and must be relatively long to sufficiently compensate for chromatic dispersion. For example, if an optical fiber is 100 km in length, then a dispersion compensation fiber should be approximately 20 to 30 km in length. Furthermore, dispersion compensation fiber is not available to compensate for the negative chromatic dispersion of DSF and NZ-DSF lines in the 1.3 .mu.m band.
A chirped fiber Bragg grating is a special fiber with spatially modulated refractive index that is designed so that longer (shorter) wavelength components are reflected at a farther distance along the chirped fiber Bragg grating than are the shorter (longer) wavelength components. A chirped fiber Bragg grating of this sort is coupled to a fiber communications system through an optical circulator. By causing certain wavelength components to travel longer distances than other wavelength components, a controlled delay is added to those components and opposite dispersion can be added to a pulse. Unfortunately, a chirped fiber Bragg grating has a very narrow bandwidth for reflecting pulses, and therefore cannot provide a wavelength band sufficient to compensate for light including many wavelengths, such as a wavelength division multiplexed light. A number of chirped fiber Bragg gratings may be cascaded for wavelength multiplexed signals, but this results in an expensive system.
A conventional diffraction grating has the property that different wavelengths are output from itself at different angles. By using a pair of gratings in a coupled spatial arrangement, this property can be used to compensate chromatic dispersion in a fiber communications system. In such a spatial grating pair arrangement, lights of different wavelengths are diffracted from a first grating at different angles. These lights are then input to a second grating which diffracts them a second time so as to set their pathways parallel to one another. Because the different lights travel with different angles between the two gratings, certain wavelength components are made to travel longer distances than other wavelength components. Chromatic dispersion is produced in the spatial grating pair arrangement because the wavelength components that travel the longer distances incur time delays relative to those that travel the shorter distances. This grating-produced chromatic dispersion can be made to be opposite to that of the fiber communications system, thereby compensating the chromatic dispersion within the system. However, the dispersion produced by a practical spatial grating pair arrangement is extremely small and is not large enough to compensate for the relatively large amount of chromatic dispersion occurring in a fiber optic communication system. Therefore, to compensate for chromatic dispersion occurring in a fiber optic communication system, the two gratings of a spatial grating pair would have to be separated by a very large distance, thereby making such a spatial grating pair arrangement impractical.
Accordingly, there exists a need for an improved chromatic dispersion compensator. The improved chromatic dispersion compensator should be practical for compensating for chromatic dispersion accumulated in an optical fiber, and should be readily adaptable to either positive or negative chromatic dispersion, and which can provide non-uniform dispersion compensation so as to compensate for fiber dispersion slope so as to accurately compensate chromatic dispersion in each of the WDM channels throughout a wide wavelength range. The present invention addresses such a need.